Comment: Is it true that actual scientists, not just bloggers and middle aged moms sending forwards, have found we share 50% of our DNA with bananas? Everywhere I look says something about a professor named Steve Jones, but I can't find out more about him

It would not be surprising - we have the same DNA code, the same Krebs cycle, the same cellular respiration, etc. etc. Most of the biological functions of any living creature is done within the cell, and plant and anaimal cells differ only in a few significant respects - cell walls, plasmids, etc. It does not take that much for the cells to express themselves on the multicellular level in very different ways, just as the relatively small number of elements express themselves in a grand variety of substances. In fact, I would be surprised if most of the differences were not in the 'junk DNA' - the large amount of DNA that is unexpressed in the function and form of the individual.

A Turtle - who thought the OP was about taking 'you are what you eat' too seriously - Named Mack

Fifty percent isn't even all that much, considering any two randomized DNA sequences will have 25% match overall by chance (there are only 4 possibilities at each position). Given the constraints already mentioned in that there are a basic set of genes needed by any organism, I'd guess that 50% match is probably near the limit of "unrelatedness."

I'm a bit surprised that your questioner couldn't find anything out about Steve Jones, he's quite well know, and is a prolific writer of popular science books, indeed, the second Google result is a wikipedia page about him.

As to the figure, well its one I've heard before, but in practical terms it doesn't mean a whole lot unless it is clearly defined. Even something as simple as comparing two homologous genes gives rise to different ways of scoring similarity. How much do you penalise gaps? Are you interested in the amino-acid sequence, or the DNA sequence (Generally you are interested in the latter), are you concerned about identity, or similarity (the former looks at exact matches in amino-acid sequence, the latter considers whether they are functionally similar.)

In considering DNA sequence as a whole, you also need to think about re-organisations, its not possible just to put one sequence next to the other and tick every A, T, C and G which are the same.

However, there is a grain of truth in the 'fact.' A huge number of genes found within humans have a homologue in bananas, and indeed pretty much all the eukaryotes. The basic cell level stuff is all the same, respiration, transcription, translation, signalling cascades, transcriptional regulation, protein degradation. In many cases not only are their related genes, but they show a significant degree of conservation in their sequence.

Indeed, the degree to which any part of sequence is conserved can be hugely useful to biologists. While our understanding of the genetic code is useful for identifying genes, it doesn't do much good in identifying other important bits of DNA. By comparing humans and bananas, or indeed any two organisms, and looking for bits of non-coding DNA that nonetheless remains similar, we can work out which bits likely have a functional role.

Comment: Is it true that actual scientists, not just bloggers and middle aged moms sending forwards, have found we share 50% of our DNA with bananas? Everywhere I look says something about a professor named Steve Jones, but I can't find out more about him

Any reference to a "percent similarity" between two species has all kinds of problems.

Like Nick said, if you just pick a random place on both genomes and start comparing "sequences" you'll get about a 25% match.

But DNA "sequence" is much more than just the pattern of A,T,G and C. The chromosomal location, nearby genes, methylation, amount of "junk" DNA in the vicinity and a bunch of other things contributes to what a particular sequence actually does (which is a much more relevant thing to compare than just raw sequence).

The genomes of most higher species is roughly 90% "junk". That "junk" is very poorly conserved across species. "Non-junk" DNA, that actually codes for a protein or acts as a regulatory element, is much more highly conserved across species but at only ~10% of the genome it really doesn't contribute all that much to the overall similarity.

You often see quotes saying how similar the DNA is between two species. In most situations the number is meaningless since how similar the genomes are depends on how you define similarity. There are a couple ways to define similarity and each method is used to examine different aspects of the DNA sequences. Quotes like "a chimp's DNA is 95% identical to humans" is very misleading. That particular comparison was almost certainly done on just the coding sequences, which is only ~10% of the genome. The noncoding DNA is much less conserved across species, and even between individuals of the same species.

If you abstract the comparison one more level, to protein sequence, the similarities between species increases. Indeed I suspect most comparisons you see between humans and other primates is actually at the protein level.

But back at the genome wide level the similarities aren't all that great and measuring the similarity at that level really doesn't have all that much value.

Another thing to consider when comparing genomes is that nobody really knows the relationship between the number of genes and the "complexity" of the organism. Or, how a small change in a single gene can significantly change the gene's behavior, and potentially the complexity of the organism. The simple minded thought that "complexity" is a linear function of the number of genes is certainly wrong. Even though scientists that should know better often treat gene number as being a linear measure of complexity. When the human genome was nearing initial completion there were a fair number of scientists that thought there was something seriously wrong with the methods used because the number of genes was turning out to only be ~30,000. That isn't all that many, something like 6x more than bacteria. "Certainly humans are more than 6x more complex than bacteria" was a fairly common thought but it is erroneous because gene number is not a valid measure of the organism's complexity.

The fact is "complexity" isn't linear in the number of genes and isn't even constant for a fixed number of genes. So even if humans and bananas share 50% of their sequences that does not really say anything about the actual relatedness, or the relative complexity, of the two species.

"Relatedness" can be determined by comparing the genomes of two species but that is a much more complex analysis than simply "we share 50% of our DNA with bananas." Any quote like "we share XX% of our DNA with {insert species name}" really has no significance, especially if you don't know what was actually being compared.